In a passive optical network such as an Ethernet Passive Optical Network (EPON), a number of optical network units (ONUs) are placed in a corresponding number of offices or homes and are coupled by passive devices to a single optical line terminal (OLT) that may be placed, for example, in a central office of a telephony service provider or cable provider. This last segment in a communication network is often referred to as the “local loop.” The OLT may communicate (in both the upstream and downstream directions) with the multiple optical network units by broadcasting Ethernet packets. Each optical network unit (which is also denoted as an optical node terminal (ONT)) extracts packets based on a media-access-control (MAC) address.
EPON offers a number of advantages over other local loop high bandwidth solutions. For example, a copper-based T1 line requires numerous repeaters to continually boost the signal whereas an EPON uses passive components between the OLT and the ONUs. DSL and cable modems tend to more complex and require considerably more maintenance and have greatly reduced bandwidth as compared to optical fiber. As compared to other optical protocols such as asynchronous transfer mode (ATM), EPON uses the simpler and much more robust IEEE 802 standard, which allows for variable-sized packets. In contrast to the fixed packet size used in ATM, the variable-sized packets in Ethernet applications are much more compatible with Internet Protocol (IP) communications. Because EPON's point-to-multipoint topology is economical yet takes advantage of optical fiber bandwidth, EPON is increasingly being implemented as the local loop solution for both fiber-to-the-business (FTTB) and fiber-to-the-home (FTTH) systems.
An EPON provides a one giga-bit-per-second (Gbps) data transmission rate in both the downstream (OLT to ONU) and upstream (ONU to OLT) directions. Through time-division-multiplexing, a variety of ONUs may share the 1 Gbps bandwidth. To provide additional bandwidth, other passive optical standards such as a Gigabit-capable-passive-optical network (GPON) standard have been developed. Regardless of the particular passive optical network standard being used, there is only so much upstream and downstream traffic that may be accommodated. These bandwidth limitations may be problematic in established fiber optic networks. For example, a fiber optic network may have been implemented many years ago. At that time, a provision of six (for example) available optical fibers to each network node may have been considered more than adequate for any conceivable data traffic. However, due to the explosive development of the Internet (and its related data and video communication demands), such an established optical network may not be adequate for current or future bandwidth demands. However, construction costs to add additional optical fiber are prohibitive.
Rather than add additional cable, wavelength division multiplexing (WDM) passive optical networks (PONs) have been developed in which multiple PONs share a single optical fiber. Each PON has a single ONU that uses a particular wavelength to communicate with the OLT. The OLT thus includes a plurality of optical transceivers, where each transceiver it tuned to a corresponding wavelength pair (one wavelength for downstream traffic, the remaining wavelength being reserved for upstream traffic). In the downstream direction, a first multiplexer/demultiplexer (Mux/Demux) multiplexes the downstream data traffic from the OLT onto the optical fiber. A second Mux/Demux demultiplexes these downstream frequencies to the corresponding optical network units (ONUs). In the upstream direction, the second Mux/Demux multiplexes the upstream frequencies from the various ONUs onto the optical fiber whereupon they are then demultiplexed by the first Mux/Demux for distribution to the corresponding transceivers in the OLT. It may thus be seen that conventional wavelength division multiplexed PONS are point-to-point systems: Each ONU has its own frequency band for upstream and downstream traffic with the OLT such that each OLT/ONU pair forms its own individual PON.
Although conventional WDM-PON systems thus multiplex several PONs onto a single optical fiber, the point-to-point nature of these systems limits communication flexibility. Accordingly, there is a need in the art for improved PON systems that increase the number of ONUs that may be serviced with regard to a given optical fiber.